Mechanical enclosures for high performance tape drives have been made of metal to insure accuracy and rigidity necessary to precisely position the cartridge. In prior tape drives where data recording density requirements have not been as high or demanding, the rigidity and the control of the locating dimensions and locating surfaces of the drive and drive housing are not as in high performance tape drives because the positioning of the magnetic tape cartridge and tape, and the magnetic heads are more forgiving for lower density recording.
With the advent of servo controlled read/write head positioning and the introduction of a cartridge having magnetic tape with servo control tracks recorded on the tape, the requirements for precision in the locating surfaces and for positioning the cartridge relative to the components of the tape drive which interface with the tape have become considerably more restrictive in order to make cartridges which are recorded on one drive readable on another drive and to insure reliable reading and writing operations in all drives.
One will appreciate that the manufacture of precision parts is well within the capability of present manufacturing skills but is very costly due to the degree of accuracy required. Typically, frames of precision tape drives have been fabricated from metal and are well known but present cost and manufacturability problems exist. The metal frames can be reduced in weight, but the rigidity and stability is compromised with continued reduction in the amount of material within the frame itself. Further, when the drive is assembled into the computer or other cabinet of the host equipment, the attachment of a tape drive enclosure with insufficient rigidity may result in distortion sufficient that the actuator may be deviated from a design position and thus degrade the operation or reliability of the tape drive.
The deformation which is so detrimental to reliable and repeatable tape drive operation can be a result of forces exerted on parts of the tape drive during installation in the host computer, assembly of parts to form the tape drive, or thermal stresses resulting from ambient temperature change or temperature rise during the operation of the device. With the trend toward making devices that rely on snap-together and non-fastener assembly wherever possible, the stresses of forcing components into snap-in retainers creates stresses in the assembly which may migrate and cause undesirable distortions which in turn may result in unreliable operation or in the failure of a high performance tape drive.
Further, because the positioning of the magnetic tape cartridge at a precisely defined and controlled position relative to the remainder of the tape drive is required for repeatability and reliability, distortions cannot be tolerated within the tape drive frame which could effect the positioning of the datums against which the magnetic tape drive cartridges are located.
The use of plastics for the enclosure is highly desirable due to cost saving and ease of manufacturing when compared with precision made machined metal parts. The use of plastics, a typical and preferred cost reduction approach, has not been particularly successful in high precision tape drives because plastics easily distort and may render the device unreliable. In order to secure adequate rigidity of the frame of the tape drive, any frame manufactured of plastics must be of such bulk as either to prevent the tape drive from accepting the cartridge or to prevent the tape drive from fitting within the required form factor.
While prior metal frames and supports in the tape drive housings provide adequate protection against electromagnetic or electrostatic discharge, metal frames have those disadvantages as described. Use of plastic materials in the chassis and/or bridge requires an approach to insure that the electromagnetic or electrostatic discharges which are occasioned by static charge build-up on the cartridge or tape do not damage the electronic components of the read/write head or circuits associated with the tape drive. A static charge buildup may be either the result of a person having a static charge on his body handling the cartridge or from spooling the tape within the cartridge and drive.